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Comment by bigyabai | Hacker News Reader

The service ceiling of the SR-71 is still well beneath the altitude of a modern balloon. The speed isn't necessary, and it's altitude record has been beaten by other modern jets.

If we're spitballing solutions, balloons are slow, fragile and move in relatively predictable paths. An F-15 could reach the same altitude, and launch it's own balloon with a small solid-fuel booster that also corrects the trajectory. If the interceptor balloon could get within 1-2km of the target balloon, then a cheap kinetic weapon firing HEI projectiles might be enough to destroy it.

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The F-15 can climb 30,000 feet.

https://en.wikipedia.org/wiki/McDonnell_Douglas_F-15_Eagle

That is the lower bound of what modern high-altitude balloons can reach. Back in 2023 multiple balloons were observed much higher than that up to 50,000 feet.

https://en.wikipedia.org/wiki/List_of_high-altitude_object_e...

This is still in the lower spectrum for such balloons. BS 13-08 reached 176,000 feet back in 2013 (there is reason to believe spy balloons would not go that high though).

https://en.wikipedia.org/wiki/High-altitude_balloon

So, you're wrong. There is no US aircraft that can reach them (unless they drop low as they did in 2023). The US likely wants to hit them before they go low.

The SR-71 is uniquely suited for this task, having reached up to 90,000 feet during testing. It was specially made for some kind of hybrid space war possibility (not balloons, but something high up there).

You misread your source: the F-15 can climb 30,000 feet in 60 seconds - the service ceiling is closer to 60,000 feet officially, and 90,000 under ideal conditions:

https://theaviationgeekclub.com/how-the-f-15-streak-eagle-br...

By "same altitude" I was referring to the SR-71. I do not know what the payload capacity was for either aircraft in the 90,000ft testing conditions, but for interception missions there are more important metrics.

Per the first link you provided, the F-15 has the following:

Service ceiling: 65,000 ft (20,000 m)

That's high enough to go 15k feet above the highest ballon mentioned in the second link you provided, which was 50,000 feet.

Also, the SR-71 carried no weapons, only cameras, and re-tooling to support them would be far more expensive than merely shooting a balloon with a missile from the ground (or building an air-to-air missile that can be fired from existing platforms at altitudes like 65k feet).

Fair correction on the altitude (still in the lower bounds of the balloons, as I mentioned).

The SR-71 already flew in the configuration I mentioned. It's the YF-12C (a converted SR-71).

Recovering and converting another sure is more expensive than one missile, but it's a reusable platform.

A ground missile able to reach those balloons is more expensive than a modernized SR-71 sortie. High-altitude, precision missiles are also not allocated for this task. China can launch dozens of balloons for cheap, and spending one multi-million-dollar missile on each is insanity. Even the long range air-to-air ones are too expensive for that.

The SR-71 (as a new YF-12D interceptor) makes more sense. It can take multiple balloons in one flight with cheap shorter range air-to-air missiles (or maybe even cheaper weapons, for a balloon a laser would do), then go home and do that over and over again.

China can launch a lot more than a dozen balloons, perhaps thousands if convincing decoy payloads can be made. The A-12 was intended to counter high-value targets in a limited number, not low-value targets en-masse.

I can't rule out that Issacman does have this mission profile in mind, but it seems contrived. Whatever weapon the SR-71 could carry to-altitude could be be done cheaper, more numerously and probably more quickly too with F-15s. Refurbishing a fleet of SR-71s and a fleet of KC-135Qs would be a lot of work for an extremely limited capability.

F-15s are not stealth. You don't want to risk escalation by sending fighter jets towards someone else's airspace. To me, they don't make sense for this kind of mission.

It does sound like a lot of work, but that's the part of the puzzle that is not a mystery. Someone is in fact putting one of those back to work, we just don't know why.

I'm missing something - why would you be wanting to intercept balloons over someone else's airspace? The 2023 incident was in US (and Canadian) airspace.

Sorry, I should have explained it: you want to intercept a spy balloon before it enters your airspace, not after it has already entered and possibly gathered any information.

Well there's a whole lot of ocean one doesn't need to be stealth to fly over but really, your comments read like endless goal-post shifting from someone who has a preferred solution in mind and is working backwards to justify it. Also the SR-71 was never stealth, only reduced observability. The USSR was very aware of the overflights taking place.

It seems no one else offered any other concrete uses that would justify the efforts that went public regarding the frame reuse.

That likely concentrated the discussions towards this single idea I presented. However, I'm not actually that committed to it. If you have any other, I would very much would like to hear about it.

It's boring, but "aerial hypersonic testbed" is almost certainly NASA's intention, just like they did in the 1990s: https://en.wikipedia.org/wiki/Linear_Aerospike_SR-71_Experim...

This is a NASA/JPL project; there are a limited number of airworthy SR-71s, they are not modernized for combat, and they cannot scramble to intercept. The M-21 variant has a well-documented conversion to a mothership[0] that can ferry payloads externally to-altitude and at high speed. This is incredibly ideal for testing captive ramjet/scramjet technology, since they require Mach 3+ speeds to attain ram pressure through the engine.

Sending the SR-71 into denied or uncontrolled airspace seems like a terrible idea, in 2026. Chinese and Russian naval interceptor missiles could shoot down the plane at it's maximum speed and altitude; it's not going up against 1980s missiles anymore. The risk is absurd, and there's not enough SR-71s to treat them like they're expendable.

[0] https://en.wikipedia.org/wiki/Lockheed_D-21

Thanks, that is a little bit more elaborate than the information on the interview and actually makes sense.

There has been some speculation about this outside this thread though:

https://theaviationist.com/2026/09/23/nasa-chief-teases-myst...

I don't know how credible those reports are, but Jared Isaacman's comments do suggest more than just a testbed. Do you have any comment on that?

I don't have access to any privileged information, here. My best bet is that DARPA is lighting a fire under JPL's ass to explore variable-cycle engine technology: https://en.wikipedia.org/wiki/Variable_cycle_engine

The US has tested ramjets before, but mostly in the context of missiles and boost-glide vehicles. The "holy grail" of efficient propulsion would be to taxi/accelerate with a turbofan engine, bypass into a ramjet for Mach 3-to-6ish, and then transition to a liquid-fuel rocket motor for the last Mach 6-10 climb. An engine like that could escape the Earth's atmosphere without sacrificing efficiency at lower altitudes. I realize that "space fighter engine" is also a longshot, but China has ramped up investment in variable-cycle technology quite a bit in the past 2 years. Simply solving the turbofan-to-ramjet transition would be a huge boon for high and fast flight.

Issacman's quotes in that article suggest, to me, that he wants to reboot the SR-72 program (or something similar) with cleansheet propulsion as a feature. The SR-71 could be the engine testbed, and a future airframe could be designed around the requirements of the engine. Just my $0.02 though.

Don't worry about being precise, this is an informal speculation thread with low stakes. I don't expect anyone (especially me) to nail it.

Thanks for the insights!

Your amounts are off. that wikipedia page lists the f-15 as having a service ceiling of 65,000 feet. Perhaps irrelevant but a f-15 shot a satellite down once(not relevant because that is more the missiles capabilities than the planes, which is probably the relevant point)

Unit mix up?

The f-16 however, does have a surprisingly low ceiling, there are fighter pilot memes about it. https://www.youtube.com/watch?v=koEQp8ImlnI?t=643 something about having small wings, get above 30,000 feet and it's not happy.

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